The subject invention relates to aqueous coating compositions. More particularly, it relates to aqueous coating compositions capable of giving a low gloss pigmented protective finish to metal coil.
Metal coil coating compositions based on polymeric film-forming materials are well known. Such compositions typically contain a film-forming material for providing a protective finish, flatting agents for imparting a desired degree of gloss and pigments. Most prior art compositions formulated for use on metal coil have been organic solvent based. However, in recent years there has been a desire to reduce or eliminate the content of organic solvents in coating compositions. Part of the reason for this has been because of the high cost of the organic solvent itself, as well as the high cost of equipment needed to remove the organic solvent from the surrounding atmosphere following a coating operation.
There is a need for aqueous coating compositions capable of giving the same set of desired properties as an organic solvent based composition. One particular problem encountered with aqueous coating compositions in the field of metal coil coating has been metal marking. Metal marking is the undesired phenomenon observed during fabrication of coated metal coils. The intentional or accidental sliding of one coated metal coil over another coated metal coil (for example, during the installation of aluminum siding on a house) often times will leave a dark mark which is difficult to remove. Metal coil coated with organic solvent based coating compositions have not been adversely affected by a metal marking problem; this is because of the fact that polymeric film formers used are capable of adequately protecting the metal coil. Metal marking has been observed to be much more acute with aqueous coating compositions, and in particular with latex coating compositions. It is believed the metal marking results from conventional flatting agents and pigments typically included in the coating compositions. Because of the way a latex film is formed, the pigment particles and flatting agents protrude through the surface of the formed film and, in effect, act as abradants. Films formed from organic solvent based compositions appear to form in such a manner that the pigments and flatting agents are buried within the film and do not protrude through the surface, thereby lessening the tendency of metal marking.
Various attempts have been made to eliminate or reduce the metal marking resistance observed with aqueous coating compositions used on metal coil. Attempts at reducing the pigment particle size, as well as using less hard flatting agents, have not been fully successful.
There have now been found aqueous coating compositions containing film former, flatting agent and pigment, which are capable of providing a protective finish to metal coil without the problem of metal marking.
As used herein, all percentages and ratios are by weight unless otherwise indicated.